Originally posted by madison_wi_gal Well, 3 gone already, but I'm considering bringing the 18-250 on a road trip as my sole lens.
I had the Tamron 18-250 for about 6 years. It's wonderfully convenient, and really quite good, particularly in good light, and particularly around 24-70mm. People who say it's no better than a phone or a p&s are talking through their hats. I found that if you shoot RAW (unfortunately I didn't when I paired it with a K100D S, before I got my K-30) and are prepared to work on the images in PP, distortion and CA was often fixable and there was usually quite a lot of resolution to work with.
But across their common range, the 18-135 is significantly better. At the long end the 18-250 is not a patch on the 55-300, and focus breathing with the Tamron means you get very modest magnification of subjects not too far away (particularly in contrast with the screw-driven versions of the 55-300, which are not internal focus lenses). The combination of 18-135 and 55-300 is far superior, in my experience.
Ultimately it depends on how much value you place on the convenience (and reduction in weight and bulk) of having a single lens over the superior performance (and WR) of the 18-135 + 55-300 combination. (I should add that I found my K-30s to be demons for getting dust on the sensor, so fewer lens changes had an additional advantage.)